As wavelength division multiplexed (WDM) networks containing large numbers of wavelength channels are becoming more common, the need for multiwavelength sources is increasingly important. Multiwavelength sources, such as tunable lasers, have the ability to tune to different wavelengths either continuously over some allowable range or at discrete wavelength values. Since each channel in a WDM optical communication system operates at a distinct wavelength, multiwavelength sources are essential to relieve inventory and stockpiling issues associated with systems with a discrete source for each wavelength.
Fast tunable lasers with switching times smaller than 50 ns promise to be attractive components for next generation optical networks. A major class of tunable lasers is formed by integrating a gain section with one or multiple tunable grating sections. In such a laser, the wavelength is switched by injecting current into one or more of the grating sections. However, these lasers typically require that the tuning current be controlled within +/−25 μA in order to obtain accurate wavelength settings. This tends to inhibit fast tuning and/or limit wavelength accuracy. Temperature changes in the grating sections during the tuning cycle further complicates the operation of these devices.
Digitally tunable lasers are formed by integrating an amplifier array with a passive wavelength selective element such as an arrayed waveguide grating (AWG) router or a diffraction grating. In these devices, the wavelength is switched by turning respective amplifiers on or off. The operating wavelength is determined by the passive element and, unlike the fast tunable lasers previously mentioned, is virtually independent of the current through the amplifiers.